Alanine transaminase (ALT) [EC 2.6.1.2., also known as glutamate pyruvate transaminase (GPT) and alanine aminotransferase] is a pyridoxal enzyme catalyzing reversible transamination between alanine and 2-oxoglutarate to form pyruvate and glutamate. By mediating the conversion of these four major intermediate metabolites, ALT plays an important role in gluconeogenesis and amino acid metabolism. In muscle and certain other tissues, ALT degrades amino acids for fuel, and amino groups are collected from glutamate by transamination. ALT transfers the α-amino group from glutamate to pyruvate to form alanine, which is a major amino acid in blood during fasting. Alanine is taken up by the liver for generating glucose from pyruvate in a reverse ALT reaction, constituting the so-called alanine-glucose cycle. This cycle is also important during intensive exercise when skeletal muscles operate anaerobically, producing not only ammonia groups from protein breakdown but also large amounts of pyruvate from glycolysis.
ALT activities exist in many tissues, including liver, muscle, heart, kidney, and brain. Molecular cloning of the complementary DNAs (cDNAs) of two human ALT isoenzymes, hALT1 and hALT2 have been disclosed in International Publication WO 02/092768, herein fully incorporated by reference in its entirety. The independent DNA encoding for the two human ALT isoenzymes (gpt1 and gpt2, respectively) has been shown to be localized to separate chromosomes in humans, and that they have distinctive tissue distribution patterns, suggesting a tissue-dependent role for ALT isoenzymes.
Perhaps the most well-known aspect of ALT is that it is used clinically as an index of liver integrity or hepatocellular damage. Serum ALT activity is significantly elevated in a variety of liver damage conditions including viral infection, alcoholic steatosis, nonalcoholic steatohepatitis (NASH), and drug toxicity, although the underlying mechanism is generally not well understood. While low level of ALT is present in peripheral circulation because of normal cell turnover or release from nonvascular sources, the liver has been shown to contain the highest levels of ALT. The difference between ALT levels in liver and in blood has been shown to be about 2,000-3,000-fold. Hence, the increased ALT in serum, plasma, or blood is regarded as a marker of liver injury because of the “leakage” of hepatic ALT into the circulation. Usually, the nature of liver injury causes the blood ALT levels to vary greatly. Extremely high transaminase levels (greater than 8- to 10-fold normal) can indicate acute viral hepatitis and/or drug-induced hepatotoxicity. A mild chronic increase of serum ALT (2- to 8-fold) is generally a characteristic of chronic hepatitis, fatty liver, and/or steatosis. However, many details of the mechanism for the correlation of ALT levels with the etiology of liver damage remain to be understood.
Even though serum ALT is one of the most widely-used assays in clinical chemistry, there are serious deficiencies with the assay because it is an inadequate predictor in some cases. Recent studies have cast doubt on serum ALT assay's specificity for liver disease. Higher than normal ALT levels are frequently associated with other clinical conditions such as obesity, muscle disease, heart failure, hemochromatosis, Wilson's disease, α1-antitrypsin deficiency.
There is a need for improved ALT immunoassays that more accurately indicate and/or diagnose tissue injury and/or disease. There is a need for an ALT animal model for research and testing purposes. There is also a need for improved animal ALT immunoassays for use in, for example, drug testing and toxicology studies.